Electrostatic color printers/plotters, usually referred to in this application as simply printers, work by passing a print medium, typically MYLAR.RTM. plastic film or paper, over or under a series of charged stylii of an electrostatic charging head. This creates small charged areas on the paper. Subsequently a toner is applied to the paper, the toner adhering firmly to the charged areas. Excess toner is removed by a vacuum chamber downstream of where the toner is applied.
Electrostatic color printers/plotters can be constructed in several ways. One system uses a single charging head and three or four toner stations, one for each color. (Although theoretically three colors, yellow, cyan and magenta can be combined to form black, to get a better black a separate black toner station is often used.) The paper is passed through the printer once for each color to be applied to the paper. Therefore, these printers require the paper to be cycled through the printer four times. This, of course, is quite time consuming. Also, the paper has a tendency to become misaligned so unless such misalignment is compensated for, image quality can be impaired.
One way to eliminate some of the problems associated with the multiple-pass color printers is shown in U.S. Pat. No. 3,523,158. The printer shown there has a single electrostatic charging head and three serially-aligned toner positions. The charging stylii in the charging head are arranged in groups of three, one for each of the primary colors. The downstream toner stations do not provide toner to the entire width of the paper but only to narrow bands corresponding to the positions of the stylii for the corresponding color. Thus, if one were to print all three colors simultaneously, one would end up with very narrow strips or bands of the three colors, alternating red, green, blue, red, green, etc. While this solves many of the problems of the multiple-pass printers, image quality is necessarily limited.
Another type of single-pass electrostatic color printer/plotter uses a number of print stations, each with its associated paper drive. It has been found necessary to use several paper drives because of the large drag exerted on the paper as it travels along its circuitous path through each print station. However, providing separate paper drives for each print station increases the mechanical complexity, the size of the printer and the complexity of the control circuitry. Furthermore, the paper is prone to stretching when traversing such paths. An example of such a printer/plotter is shown in U.S. patent application Ser. No. 722,497, entitled "Single-Pass Color Plotter" filed on Apr. 12, 1985.